fix crocodoc bug in speedgrader

fixes CNVS-13920, fixes CNVS-13447

Test plan:
  This needs to be tested against two different HEADs D:
  (6318ca161 and master)

  * Go to speedgrader in an assignment that has crocodocable submissions
  * Make sure the submission loads in crocodoc
  * Assign grades to a student
  * The submission should still be displayed in Crocodoc
  * The submission dropdown should work correctly

Change-Id: If42efff4b8f6f9301fc1a5dbe5c4f52caf9a295c
Reviewed-on: https://gerrit.instructure.com/37419
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>